A feedthrough capacitor is a limited inductance conductor with a known threshold of capacitance that is typically used to carry a signal through the grounded metal chassis, or panel, of an electronic component. In the art, feedthrough capacitors are commonly incorporated into power supply leads for electronic components with high frequency circuitry. In this application, the feedthrough capacitor is designed to deliver DC or lower frequency alternating current (AC) signals to the electronic component while, at the same time, bypassing relatively high frequency alternating current (AC) signals, such relatively high radio frequency (RF) energy, to the grounded metal chassis. In this capacity, the feedthrough capacitor acts as a very low inductance filter that prevents the potentially harmful, high frequency AC signals from being transferred into or out of the electronic equipment along the power supply lead.
A feedthrough capacitor traditionally comprises a capacitor through which a lead, or center electrode, is passed, the lead being conductively coupled to one terminal of the capacitor. The other terminal of the capacitor is typically conductively coupled to a collar-shaped housing, or outer electrode, that is in turn transversely mounted within a fitted opening formed in the metal chassis. As such, relatively high frequency electrical energy carried by the lead is diverted by the capacitor to the grounded metal chassis by means of the outer electrode.
In the art, the particular design of the capacitor through which the lead is passed often varies considerably both in its overall geometry, or style, as well as its dielectric material composition.
For instance, one type of feedthrough capacitor which is well-known in the art utilizes a tubular ceramic capacitor. Specifically, a hollow, ceramic tube is utilized as the dielectric material, the inner and outer surfaces of the tube being metalized. In this manner, the capacitance of the tubular ceramic capacitor is largely defined by the thickness, diameter and dielectric properties of the ceramic material. To complete manufacture of the feedthrough capacitor, an inner, or through, electrode is coaxially passed through the longitudinal bore defined by the capacitor and is conductively coupled to its metalized inner surface. Finally, an outer mounting collar, or outer electrode, is coaxially disposed over the ceramic tube and is soldered or otherwise conductively coupled to its metalized outer surface.
Another type of feedthrough capacitor which is well-known in the art utilizes a discoidal ceramic capacitor. Specifically, an annular ceramic body is formed with metallized inner and outer surfaces. Similar to a tubular capacitor, the inner surface of a discoidal capacitor is conductively coupled to a lead passed therethrough and the outer surface of a discoidal capacitor is conductively coupled to collar-shaped housing that is adapted for mounting within a metal chassis. As a unique part of its internal construction, a discoidal ceramic capacitor includes multiple, spaced apart, overlapping metal layers that alternately connect to the inner and outer metallized surfaces. The inclusion of the overlapping metal layers significantly increases the total surface area between opposing metal surfaces, thereby rendering discoidal ceramic capacitors with a higher capacitance per unit volume construction than tubular ceramic capacitors.
Although well-known in the art, feedthrough capacitors of the type as described above that rely upon a ceramic dielectric material have been found to suffer from a few notable shortcomings.
As a first shortcoming, ceramic materials are inherently fragile by nature. Because both tubular and discoidal ceramic capacitors rely upon the ceramic dielectric for significant structural support, it has been found that the ceramic dielectric in each design is rendered highly susceptible to cracking, fragmentation or the like. In particular, irreversible damage to the ceramic dielectric is often caused from forces applied thereto during routine assembly and installation of the feedthrough capacitor as well as from exposure to changes in temperature.
As a second shortcoming, it has been found to be rather difficult to construct ceramic-type feedthrough capacitors in larger form factors. As a result, the capacitance of most ceramic-type capacitors is rather limited. Furthermore, due to the aforementioned size restrictions, the diameter of the capacitor through hole (i.e., the hole through which the lead is passed) is generally limited. Consequently, the size of the center electrode that passes through the capacitor is similarly restricted, thereby limiting its current carrying capability.
In view of the aforementioned shortcomings associated with feedthrough capacitors that rely upon a ceramic-based dielectric, feedthrough capacitors are also commonly constructed using a dielectric film constructed from one or more layers of a polymer material, a paper material, or a composite thereof. As part of its manufacture, the opposing faces of the dielectric film are applied with a metal, such as aluminum. The resultant dielectric film is then wound around a hollow insulating tube, or other similar structure, to enable the lead to pass therethrough.
Feedthrough capacitors that rely upon a polymer and/or paper-based dielectric film have been found to be desirable in that a relatively high capacitance level and high voltage carrying capabilities can be obtained. However, at the same time, the use of a polymer and/or paper-based dielectric film has been found to be subject to numerous performance disadvantages including, but not limited to, a considerable sensitivity to high temperature environments as well as a significant size requirement that limits its use in smaller applications.